# std::experimental::sample

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< cpp‎ | experimental

Defined in header `<experimental/algorithm>` |  |
template< class PopulationIterator, class SampleIterator, class Distance, class URBG > SampleIterator sample( PopulationIterator first, PopulationIterator last, SampleIterator out, Distance n, URBG&& g ); | (1) | (library fundamentals TS)
template< class PopulationIterator, class SampleIterator, class Distance > SampleIterator sample( PopulationIterator first, PopulationIterator last, SampleIterator out, Distance n ); | (2) | (library fundamentals TS v2)

Selects n elements from the sequence `[`first`,` last`)` such that each possible sample has equal probability of appearance, and writes those selected elements into the output iterator out.

If n is greater than the number of elements in the sequence, selects last - first elements.

The algorithm is stable only if `PopulationIterator` meets the requirements of LegacyForwardIterator.

1) Random numbers are generated using the random number generator g.
2) Random numbers are generated using the per-thread engine.

### Parameters

first, last | - | pair of iterators forming the range from which to make the sampling (the population)
out | - | the output iterator where the samples are written. Must not be in the range `[`first`,` last`)`
n | - | number of samples to make
g | - | the random number generator used as the source of randomness
- `PopulationIterator` must meet the requirements of LegacyInputIterator.
- `SampleIterator` must meet the requirements of LegacyOutputIterator.
- `SampleIterator` must also meet the requirements of LegacyRandomAccessIterator if `PopulationIterator` doesn't meet LegacyForwardIterator.
- `PopulationIterator`'s value type must be writeable to out.
- `Distance` must be an integer type.
- `URBG` must meet the requirements of UniformRandomBitGenerator and its return type must be convertible to `Distance`.

### Return value

Returns a copy of out after the last sample that was output, that is, end of the sample range.

### Complexity

Linear in std::distance(first, last).

### Notes

This function may implement selection sampling or reservoir sampling.

### Example

Run this code

    #include <experimental/algorithm>
    #include <iostream>
    #include <iterator>
    #include <random>
    #include <string>

    int main()
    {
        std::string in = "abcdefgh", out;
        std::experimental::sample(in.begin(), in.end(), std::back_inserter(out),
                                  5, std::mt19937{std::random_device{}()});
        std::cout << "five random letters out of " << in << " : " << out << '\n';
    }

Possible output:

    five random letters out of abcdefgh : cdefg

### See also

random_shuffleshuffle (until C++17)(C++11) | randomly re-orders elements in a range (function template)

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